This invention relates to the treatment of plastic sheet and, more specifically, to the treatment of a co-extruded layer or sheet of orientable plastic film comprised of foam and film layers which are continuously stretched.
1. Field of the Invention
The physical properties of certain plastic materials are significantly improved by cold-stretching the material to impart a mechanical stress thereto. Filaments of crystalline-type plastic materials such as nylon, saran, polyethylene, and the like, are commonly cold stretched by amounts up to 400% or more after extrusion. Such treatment has frequently been found to increase tensile strength of the filament by as much as eight to ten times. Similar increases in physical properties have also been attained in cold stretching sheet or film to improve the mechanical stress.
Continuous stretching of the sheet in the longitudinal direction alone has been found to impart highly directional properties to the sheets which in many applications of use may be found undesirable. When sheets are cold stretched in the longitudinal direction, the tensile strength will ordinarily be increased by amounts of 400% or more, as in the case of filaments, but will remain constant or only slightly increased in the transverse direction. In some processes, in order to remedy the difficulty of directional properties, sheets have been stretched laterally as well as longitudinally.
In the case of lateral stretching of a continuously-moving sheet, problems are encountered in continuously gripping the edges of the sheet while it is moved forwardly in a longitudinal direction. Pairs of diverging conveyor belts have been used to grasp the sheet edges in order to stretch it laterally. Proper gripping of the sheet edges has been difficult to achieve so that frequently non-uniform stretching is imparted to the sheet, resulting in wrinkling and uneven stressing. Such sheet edges at spaced points have also exhibited puckering or wrinkling of the sheet.
2. Description of the Prior Art
U.S. Pat. No. 3,536,797 to Cowan et al discloses a method for extruding a foamed polyolefin having a high gloss surface finish. The foamed molten film is quenched, heated to the orientation temperature of the thermoplastic on a smooth uniform heating surface, and then stretched along its longitudinal axis in heated condition to impart the desired orientation.
U.S. Pat. No. 3,076,232 to Dengler relates to a process for orienting ethylene film by stretching in a lengthwise direction at a temperature just below the melting temperature of the film, and then subjecting the film to a second stretching step at a higher temperature than the first stretching step but below the film melting temperature.
U.S. Pat. No. 3,558,764 to Isaacson et al discloses a process for preparing a microporous film wherein the film is cold drawn at ambient or near-ambient temperature and heat set.
U.S. Pat. No. 3,422,172 to Dekher discloses a process for removing corrugation from the surfaces of a plastic foam sheet as formed by slightly stretching the surfaces in a longitudinal direction.
U.S. Pat. No. 2,778,057 to Clark discloses method and apparatus for continuously stretching plastic film in a lateral direction using two pairs of rollers disposed on opposite sides of the sheet to grip the sheet as it is moved forwardly in a lengthwise direction.